1. Field of the Invention
The present invention relates to a lens apparatus, camera system and camera provided with an autofocusing function.
2. Description of the Related Art
There are many camera systems provided with an autofocusing (AF) function which detects a focusing state of an image-taking optical system using a focus detection sensor and moves a focus lens to an in-focus position using an actuator according to the detected signal. There are also a variety of autofocusing systems.
For example, a camera system which performs an AF operation based on a TTL phase difference detection system disclosed in Japanese Patent Publication No. H4 (1992)-10051, repeatedly performs focus detections not only before starting the driving of the focus lens but also during the driving of the focus lens, so-called overlap control, in order to shorten the time required to reach an in-focus state and moves the focus lens to the in-focus position while correcting the amount of driving of the focus lens based on the result.
However, with the trend toward implementation of an image-pickup element with a multitude of pixels and a smaller in size in a digital camera system in recent years, there is a growing demand for an AF function with a higher degree of accuracy and the above described AF has problems as shown below.
For example, in a system which corrects an amount of driving of the focus lens while carrying out overlap control, what determines the accuracy of stopping at an in-focus position finally is the final focus detection operation and correction of the amount of driving of the focus lens which are carried out immediately before the driving of the focus lens is completed. As the position of the focus lens at this time becomes closer to the in-focus position, it is possible to reduce errors in focus detection due to influences of aberration variations, etc., and calculation errors in the amount of driving due to variations in focus sensitivity (amount of movement of a focal point with respect to a unit amount of movement of the lens) and thereby improve the accuracy.
According to the AF system disclosed in the above described Publication, the focus detection operation during the driving of the focus lens is carried out at constant intervals, and therefore the position of the focus lens when the focus detection operation is carried out immediately before the driving is completed changes according to the amount of driving of the focus lens based on the initial focus detection operation, resulting in a problem that the accuracy of focus detection and accuracy of calculations of the amount of driving of the lens are not stable.
Furthermore, in a recent camera system capable of faster driving of the focus lens in order to reduce the focusing time, there are problems such as deterioration of focus detection accuracy due to an image flow on a focus detection sensor caused by the driving of the focus lens and deterioration of accuracy of focus detection and accuracy of calculations of the mount of driving where the focus detection operation immediately before the driving is completed is considerably far from the in-focus position.